1. Field of the Invention
This invention relates to guided projectiles that engage targets by detecting and following laser light scattered from the targets, and more particularly to a test station for calibration of the spatial transfer function (STF) of the semi-active laser (SAL) seeker.
2. Description of the Related Art
Laser guided ordinance is commonly used to engage point targets with a high probability of success and minimal collateral damage. Such ordinance includes guided artillery projectiles, guided missiles, and guided bombs, all of which will be referred to herein as “projectiles”.
A laser guided projectile's guidance system typically includes a semi-active laser (SAL) seeker, fixed-post or gimbaled, to detect pulsed laser electro-magnetic radiation (EMR) scattered from the intended target and to provide signals indicative of the target bearing and a flight controller that processes the signals to manipulate one or more control surfaces (e.g. fins or canards) to guide the projectile to the target. The SAL seeker includes a non-imaging optical system that captures and focuses the scattered laser EMR into a spot onto a segmented non-imaging detector (e.g. a quad-cell detector) or imaging detector. As the target bearing changes the position of the spot on the detector changes. The detector compares the integrated EMR incident on each cell (segment) to calculate a spatial displacement of the centroid of the spot. The effective field-of-view (FOV) is dictated by the central monotonic region of the detector's spatial transfer function (STF) in which the spot is incident on all four cells, which is in turn determined by the spot size. The detector's central monotonic region is commonly referred to as the “linear” region. The seeker maps the spatial displacement ΔX and ΔY along orthogonal axes to Azimuth (Az), Elevation (El) angles in a calibration table to select an angle measurement as an estimate of the bearing to target.
Referring now to FIG. 1, a laser guided projectile 100 may engage a target 190 by detecting and following scattered laser radiation 195 from the target 190. In FIG. 1, the target 190 is represented as a tank, but may be another type of vehicle, ship, boat, or a structure, building or other stationary object. The target 190 may be illuminated with laser radiation 185 from a laser designator 180. The laser designator 180 may be located on the ground, as shown in FIG. 1, or may be located in a vehicle, ship, boat, or aircraft. The laser designator could be located on the projectile itself. This is typically referred to as an active laser seeker. The scattered laser radiation 195 is a portion of the illumination laser radiation 185.
The laser-guided projectile 100 may include a projectile body 115, control surfaces 125, and a guidance system. The guidance system may include a SAL seeker, of which only a transmissive dome 132 is visible in FIG. 1. The guidance system may include a flight control system to control the flight of the laser guided projectile 100 by manipulating one or more control surfaces 125 based on at least one guidance signal from the SAL seeker. In the example of FIG. 1, the control surfaces 125 are shown as canards, but may be fins, wings, ailerons, elevators, spoilers, flaps, air brakes or other controllable devices capable of affecting the flight path of the laser guided projectile 100.
FIG. 2a shows a frontal view of a detector 250 and a focused laser spot 245. The detector 250 may comprise a “quad-cell” detector including four quadrants or “segments” A, B, C, D. Other detector configurations including multiple segments may be used. Each quadrant may produce a corresponding signal A, B, C, and D in response to the integrated laser power incident upon each quadrant. Guidance signal ΔX may indicate an imbalance between the laser power incident upon the left (quadrants A and B) and right (quadrants C and D) halves of the detector 250. Guidance signal ΔY may indicate an imbalance between the laser power incident upon the top (quadrants A and C) and bottom (quadrants B and D) halves of the detector 250. The terms “left”, “right”, “top”, and “bottom” refer to the detector 250 as shown in FIG. 2a and do not imply any physical orientation of the detector 250 within a projectile 100. When the laser spot 245 is centered on the detector 250, the signals A, B, C, D may be essentially equal and the guidance signals ΔX and ΔY may both be zero or nearly zero.
More particularly, the detector 250 may effectively measure the centroid of the incident EMR on the detector 250. The spatial transfer function (STF) 255 is a ratio of the laser power on the different quadrants of the detector. When laser power in spot 245 is hitting all four quadrants A-D, the guidance system operates in a linear region (or more generally a “monotonic” region) 260 of the transfer function 255. Within the linear region ΔX=((A+D)−(B+C))/(A+B+C+D) and ΔY=((A+B)−(C+D)/(A+B+C+D) where A, B, C and D are integrated laser power incident on the respective cells. The transfer function 255 in the linear region 260 determines via a calibrated look-up table (LUT) the Az, El angles of the guidance system from the target (e.g. target bearing). When laser power is hitting only two quadrants, the guidance system operates outside the linear region, where the transfer function nears +/−1. The guidance system only knows the direction towards the target, but not its true angle.
The SAL seeker is calibrated offline to generate the calibration LUT that maps the measured and calculated ΔX and ΔY to the Az, El angle pairs over a field-of-view (FOV) and with an angular resolution required for a mission. The SAL detector is mounted on a high precision 3-axis stage. A single Q-switched laser and collimator are mounted in a static fixture to direct a pulsed beam to simulate a spot reflected off a target. A controller rotates the SAL sensor on the 3-axis stage to specified locations to detect the stationary target. A computer records the measurements and maps each ΔX, ΔY pair to the Az,El pair for each location of the stage to generate the LUT. This test station and methodology is both expensive and slow. Currently, the total cost of a test station is >$350,000; $100000 for the Q-switched laser and focusing optics, $100,000 for the collimator and $150,000 for the 3-axis stage. The cost is driven by the required angular resolution of both the source and rotation of the stage. A typical medium-fidelity calibration may involve 400 measurements. For each measurement, the 3-axis stage must rotate to the specified location and settle, which takes approximately 2.5 seconds. Data acquisition requires another 0.5 seconds to detect a sufficient number of pulses (e.g. 50) at the Q-switched laser's maximum operable PRF (e.g. 100 Hz) for a high SNR measurement. The medium-fidelity calibration of a single SAL seeker requires approximately 20 minutes. A high-fidelity calibration may take up to 50 minutes. Furthermore, as both the FOV and fidelity specifications increase the calibration time will increase. In a manufacturing environment that must calibrate thousands of seekers the time requirement is burdensome and expensive. Their remains a long felt need for a more cost-effect and time-efficient method to calibrate SAL seekers.